1. Field of the Invention
The invention relates to centrifugal compressors.
2. Description of the Prior Art
A well-known arrangement of integrally geared multi-stage centrifugal compressors is the "bull gear" arrangement in which the impeller shafts are parallel to one another and have respective gears mounted thereon which mesh with a central driving gear at spaced locations around the periphery of the driving gear. In such arrangements the gearing between the central driving gear and the individual impeller shafts is selected to step up the output speed of standard 2-pole 60 Hz motors which rotate at 3,600 revolutions per minute so that the impeller shaft of the final stage rotates at a speed of up to 60,000 revolutions per minute. This speed represents a gear ratio of approximately 17:1 which is as high as can be practicably obtained at present with this type of gearing. When such an arrangement of gearing is used with a standard British 2-pole 50 Hz motor which rotates at 3,000 revolutions per minute the maximum final compressor stage impeller shaft speed is limited by present day gearing to 50,000 revolutions per minute.
As the impeller speed increases so does the pressure ratio increase at which a centrifugal compressor stage of a particular flow capacity operates efficiently. Thus it may be necessary to employ more stages of compression in areas of 50 Hz electrical supply than in areas of 60 Hz electrical supply for comparable sizes of compressor operating at the same discharge pressure and using the "bull gear" arrangement. Alternatively, it will be appreciated that this feature can restrict the range of compressor sizes that may be utilised in areas of 50 Hz electrical supply compared with areas of 60 Hz electrical supply.
It also follows that an arrangement of gearing which allows higher overall gear ratios can reduce the number of stages required for normally used discharge pressures in areas of both 50 Hz and 60 Hz electrical supply.
Any standard design of integrally geared centrifugal compressor will require different arrangements of "bull gearing" according to the frequency of the electrical supply in the area where it is used which determines the drive motor running speed.
Furthermore, "bull gear" arrangements also require a large diameter driving gear in the order of 27-30 inches diameter which results in a pitch line velocity of the gear teeth higher than 25,000 feet per minute and sometimes in excess of 30,000 feet per minute at which speeds the gears need to be very accurately machined with gear tooth profiles specifically adapted to suit each individual combination of gear load and speed. A further disadvantage of this arrangement has been the design and provision of suitable bearings for the high speed compressor shafts. Rolling element bearings are generally beyond their range of suitable application and the use of plain journal bearings has generally resulted in vibration problems through oil film instability when the impeller shafts are running unloaded at normal operational speeds. This has led to the use of relatively complicated and expensive tilting pad or special profile journal bearings.